1. Field of the Invention
The present invention relates to a method for manufacturing titanium oxide.
2. Description of the Related Art
In manufacture of titanium oxide by a known sulfuric acid process, a titanium-iron-containing substance such as a titanium-iron ore (limenite, or anatase) and titanium slag is brought into contact with concentrated sulfuric acid at an elevated temperature to produce a solid product composed mainly of a sulfated product of titanium containing ferrous sulfate, ferric sulfate, and titanyl sulfate as the main constituents together with magnesium sulfate, chromium sulfate, manganese sulfate, vanadium sulfate, sulfates of other elements, and unreacted substances. The solid is subsequently put into water or sulfuric acid solution for leaching, and then a reducing agent such as iron scrap is added there to convert the ferric sulfate to ferrous sulfate. The solution is cooled to precipitate ferrous sulfate crystal (FeSO.sub.4 .multidot.7H.sub.2 O), giving a titanium-iron solution containing titanyl sulfate in which iron content is decreased. The titanyl sulfate is hydrolyzed in such a manner that the concentrated titanyl sulfate solution is gradually added into a dilute titanyl sulfate solution at nearly a boiling state to raise the concentration of titanyl-iron sulfate and then water heated to 90.degree.-95.degree. C. is added thereto in an amount of 1/4 in volume relative to the titanyl sulfate solution.
This method is disadvantageous in that as much as 5-6% of the titanyl sulfate remains in the solution without converting to titanium hydroxide and is discarded even though the total sulfuric acid concentration in the hydrolyzed aqueous solution is 350-400 g/l H.sub.2 SO.sub.4, while 94% to 95% of the titanyl sulfate precipitates as a hydroxide.
A solvent extraction technology for titanium oxide manufacture is disclosed in U.S. Pat. No. 3,067,010. This technology relates to a selective extraction of a titanium ion species from the sulfuric acid solution by contact with an organic solvent containing an alkylphosphoric acid to separate titanium from iron in titanyl-iron sulfate in the sulfuric acid solution. This method is disadvantageous in that the iron is co-extracted in a large amount and that the 5-6N HCl for removal of the iron, and HF and NH.sub.3 for inverse extraction of Ti from the organic phase are less economical in comparison with the conventional methods of hydrolysis for titanium hydroxide manufacture.
Methods for separating titanium ions and iron ions contained in a chloride solutions are disclosed in U.S. Pat. Nos. 3,795,727 and 3,104,950. The both methods relate to extraction of iron chloride complex from an aqueous solution, and have many disadvantages such that the total HCl concentration in the aqueous solution employed have to be sufficiently high, that the extracting agents are significantly soluble in the aqueous solution, and that the titanium is coexisting with impurity metal ions other than iron, which requires further purification.
A further method was presented at the meeting of Metallurgical Society of AIME held on February 16-20, 1969 regarding selective removal of Cr (III) ions by extraction, by use of a primary amine, from a waste sulfuric acid solution containing Ti ions and Cr (III) ions for the purpose of recycling the sulfuric acid. Still further, a method is described regarding selective removal of Cr (VI) ions from a sulfuric acid solution containing Ti ions and Cr ions by use of a tertiary or quaternary alkylamine in Ind. Eng. Chem. Prod. Res. Develop, Vol. 9, No. 3, 1970. The both methods, however, relates to extraction of Cr ions and Nb ions as impurities in a form of a sulfate complex without extracting Ti ions in the sulfuric acid solution and with Ti ions kept in the aqueous solution. Accordingly, these methods are different in principle from the present invention in which Ti ion is extracted and is further purified.